It is popular opinion that the hearing capacity of man representative of vertebrates and, more particularly of mammals, is associated with active amplification processes in the internal ear which, as a whole, improve the sensitivity and frequency resolution of the ear, it being so-called otoacoustical emissions that are viewed as being the epiphenomenon of such processes. These are sound signals emitted by the ear capable of occurring spontaneously or being prompted by external stimulation, i.e. evoked, whereby the stimulation may occur, for example, acoustically or electrically.
The basic significance of evoked otoacoustical emissions for obtaining data as to hearing capacity has been known for a long time. Thus, it is described by Kemp in EP-B1-15258 that sound emissions as a reaction to a sound event can be measured in the outer auditory meatus which relate to the condition in the ear, whereby the otoacoustic emissions were measured by Kemp with the aid of an acoustical probe consisting of a highly-sensitive miniature microphone and a sound transmitter.
It is further known to determine the distortion products of otoacoustical emissions resulting from bitonal stimulation of the hearing organ, the cochlea in man, with two pure tones, so-called primary tones whereby sound emissions occur with the frequencies nf.sub.1 +mf.sub.2 having the whole numbers n and m. In man it is particularly the so-called cubic distortion product of 2f.sub.1 -f.sub.2 that permits good measurement. However, determining such DPOAE exhibits no good correlation between the amplitude of the distortion product and conventionally measured values for the hearing threshold.
By adding a third stimulus tone as the so-called disturbance tone or suppressor the DPOAE amplitude can be influenced. It is from such measurements that the conclusion arrived at in prior art was that the DPOAE amplitude can be influenced by a suppression tone in the range of the two primary tones, whereas a suppression tone close to the frequency of the distortion product has no influence on the DPOAE amplitude. In this context reference is made to the publication of Plinkert, Harris and Probst in HNO (1993) 41: 339-344 as prior art.
All attempts to date in the field of measuring otoacoustical emissions and distortion products of otoacoustical emissions have in the end the object of providing a method for objectively measuring the hearing capacity. Such a method is thus of great significance because, for example, the (subjective) indications of a human proband as a reaction to the stimulus are no longer required and measurements on all the various hearing organs can be compared to each other.